The overall aims of the proposed research program are to 1. Examine the relations between active Na transport and the electrophysiologic properties of rabbit colon using recently developed microelectrode techniques and theoretical models; 2. Develop techniques for the fabrication of sodium, chloride and potassium sensitive microelectrodes suitable for the determination of intracellular ion activities in mammalian epithelia and to apply these techniques, initially, to the examination of the relation between intracellular Na and Cl activities and NaCl transport by rabbit gallbladder; 3. Examine the relation between active transepithelial Na transport and K movements across the basolateral membranes employing pure suspension of villus cells isolated from rabbit ileum; and 4. Examine the properties of the mechanism responsible for coupling between active chloride absorption and bicarbonate secretion by rabbit colon under physiologic conditions and in the presence of cyclic 3'-5', adenosine monophosphate.